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Land-cover types were analyzed for 1970, 1990 and 2000 as the bases for determining land-use systems and their influence on the resilience of tropical rain forests in the Tehuantepec Isthmus, Mexico. Deforestation (DR) and mean annual transformation rates were calculated from land-cover change data; thus, the classification of land-use change processes was determined according to their impact on resilience: a) Modification, including land-cover conservation and intensification, and b) Conversion, including disturbance and regeneration processes. Regeneration processes, from secondary vegetation under extensive use, cultivated vegetation under intensive use, and cultivated or induced vegetation under extensive use to mature or secondary vegetation, have high resilience capacity. In contrast, cattle-raising is characterized by rapid expansion, long-lasting change, and intense damages; thus, recent disturbance processes, which include the conversion to cattle-raising, provoke the downfall of the traditional agricultural system, and nullify the capacity of resilience of tropical rain forest. The land-use cover change processes reveal a) the existence of four land-use systems (forestry, extensive agriculture, extensive cattle-raising, and intensive uses) and b) a trend towards the replacement of agricultural and forestry systems by extensive cattle-raising, which was consolidated during 1990–2000 (DR of evergreen tropical rain forest=4.6%). Only the forestry system, which is not subject to deforestation, but is affected by factors such as selective timber, extraction, firewood collection, grazing, or human-induced fire, is considered to have high resilience (2 years), compared to agriculture (2–10 years) or cattle-raising (nonresilient). It is concluded that the analysis of land-use systems is essential for understanding the implications of land-use cover dynamics on forest recovery and land degradation in tropical rain forests.  相似文献   
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The most critical aspect of polymerization reactors is the agitator's stop and becomes more serious when this occurs exactly at the removal of the heat load, which is generated by the reaction (exothermic). This phenomenon is typical in power-failure situations of the energy supply or because of other related problems at the plant. The objective of this work is to compare the performance of two configurations for the inhibition-injection system for vinyl chloride monomer (VCM) polymerization reactors. To determine the injection time a rigorous mathematical model was developed. From the results of this work one of the configurations was implemented in an industrial plant.  相似文献   
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Based on observations in animals, there is an increasing evidence that a number of persistent organochlorine pollutants can alter the endocrine homeostasis, this resulting in toxic effects in particular in the developing organism. However, the role of these chemicals in determining endocrine-related diseases in humans, and possibly a decrease of fertility, is still controversial. Exposure data concerning the human reproductive system are essential for risk assessment. Based on this, the occurrence in follicular fluid of polychlorobiphenyls (PCBs), 2,3,7,8-chlorosubstituted polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), 1,1,1-trichloro-2,2-bis(4-chlorophenyl)-ethane (p,p'-DDT) and its metabolites, and hexachlorobenzene (HCB) was investigated. With respect to PCBs, the sum of the three most abundant congeners (PCBs 138, 153 and 180) was 1230 ng/g, lipid basis (0.37 ng/g, wet weight). Congener distribution profile overlapped what is usually observed in other human tissues, as blood and milk. PCDDs, PCDFs, p,p'-DDT and 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethane (p,p'-DDD) were below their determination limits. 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethene (p,p'-DDE) and HCB were detected in concentrations respectively in the order of 700 and 70 ng/g, lipid basis (approximately 0.2 and approximately 0.02 ng/g, wet weight).  相似文献   
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Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), mono-ortho and non-ortho polychlorinated biphenyls (dioxin-like PCBs) were determined in samples of human fat tissue from nine Italian obese patients. The toxicity equivalent (TEQ) values ranged from 9 to 25pgTEQg(-1) lipid (WHO-TEF values, 2005 [Van den Berg, M., Birnbaum, L.S., Denison, M., De Vito, M., Farland, W., Feeley, M., Fiedler, H., Hakansson, H., Hanberg, A., Haws, L., Rose, M., Safe, S., Schrenk, D., Tohyama, C., Tritscher, A., Tuomisto, J., Tysklind, M., Walker, N., Peterson, R.E., 2006. The 2005 World Health Organization reevaluation of human and mammalian Toxic Equivalency Factors for dioxins and dioxin-like compounds. Toxicol. Sci. 93, 223-241]), the contribution of dioxin-like PCBs was more than 30% of the total TEQ values. The obese body burdens varied from 6 to 11ngTEQkg(-1) body weight (BW), exceeding the estimated steady-state body burden 5ngTEQkg(-1) BW, based on lipid adjusted serum concentrations from several populations in the mid-1990s, calculated in the risk assessment US EPA document.  相似文献   
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Several studies have examined the existence of recruitment communication mechanisms in stingless bees. However, the spatial accuracy of location-specific recruitment has not been examined. Moreover, the location-specific recruitment of reactivated foragers, i.e., foragers that have previously experienced the same food source at a different location and time, has not been explicitly examined. However, such foragers may also play a significant role in colony foraging, particularly in small colonies. Here we report that reactivated Scaptotrigona mexicana foragers can recruit with high precision to a specific food location. The recruitment precision of reactivated foragers was evaluated by placing control feeders to the left and the right of the training feeder (direction-precision tests) and between the nest and the training feeder and beyond it (distance-precision tests). Reactivated foragers arrived at the correct location with high precision: 98.44% arrived at the training feeder in the direction trials (five-feeder fan-shaped array, accuracy of at least ±6° of azimuth at 50 m from the nest), and 88.62% arrived at the training feeder in the distance trials (five-feeder linear array, accuracy of at least ±5 m or ±10% at 50 m from the nest). Thus, S. mexicana reactivated foragers can find the indicated food source at a specific distance and direction with high precision, higher than that shown by honeybees, Apis mellifera, which do not communicate food location at such close distances to the nest.  相似文献   
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